1. Field of the Invention
The present invention relates to a radiant heater, in particular for heating a glass-ceramic hot plate, having an area bearing an electrical heating element, with this area being the surface of an insulator which comprises a mounting layer and/or a thermally insulating microporous insulation layer, and having a dish-shaped enclosure which forms an annulus surrounding the insulator.
2. The Prior Art
German Patent 30 20 326 C2 discloses a radiant heater for a glass-ceramic hot plate in which between the electrical heating coil and the bottom of a metal receiving dish there is located a one-piece thermal insulation material which comprises, on the bottom side, a microporous insulation layer and, in the direction towards the heating coil, comprises a microporous hardened mounting layer. The heating coil is adhesively bonded on the upper side of the mounting layer. In addition, the mounting layer has an additionally densified circumferential border which projects beyond the plane of the heating coil and functions as support for the glass-ceramic hot plate. For reinforcement, the mounting layer opposite the insulation layer comprises, besides a hardener, also an increased proportion of silicate fibers.
German Patent 28 58 342 C2 describes a radiant heater for hot plates in which the electrical heating coil is stapled into an insulating plate which comprises ceramic reinforcing fibers. The staples are held in the insulating plate by friction. The plate is set in a protective pan of metal. Furthermore, there is provided a ring of reinforcing fibers which rests on the margin of the insulating plate and projects over the upper edge of the protective pan so that in the operating state the surface of the ring supports the glass-ceramic hot plate.
International Patent Application WO-91/06193 discloses a radiant heater unit having a ceramic heating-element support which covers preferably 10-20% of the area of thermal insulation arranged underneath. The thermal insulation comprises a microporous thermal insulation material having a fiber content of 0-50% by weight.
It is generally known, and recognizable from the cited patents, that the mechanically stressed components of a radiant heater, in particular the layer provided for mounting of the electrical heating element and the support of the hot plate, have to comprise a strong material which resists the mechanical stresses in assembly and installation of the radiant heater and during its operation. A disadvantage is that the thermally insulating action of the mechanically stressable materials used hitherto is either unsatisfactory because of their high density, or microporous insulation material which is reinforced with fibers has to be used. Since, besides the carcinogenic asbestos fibers, other mineral fibers have also at least become suspected of being able to cause cancer, there is increasing public pressure to replace fiber-containing materials by fiber-free analogs. On the side of industry too, there is increasingly a search for fiber-free substitutes, not least to avoid a possible liability for removal of fiber-containing material after use thereof.
A further disadvantage results from the fact that the electrical heating elements are occasionally also fixed by means of metallic staples which are driven into the thermally insulating insulation material and can possibly cause short circuits if a staple accidentally contacts the metallic wall of the dish-shaped enclosure which surrounds the thermally insulating insulation material. Similar problems with electrical safety can result if the metallic heating element is partially pressed into the insulation layer, so as to anchor it therein. In addition, for an enclosure constructed of metal, heat losses resulting from the insufficient thermal insulation of the metal have to be accepted.